There are many different laxatives for improving bowel comfort through the easier passage of stool. Some of these rely on pharmacological components to improve Taxation through stimulating contraction of the muscles of the colon wall. However, while these irritant agents improve Taxation, they do little to improve the health of the bowel in the short or long term.
The optimal laxatives are those which rely on alternatives to such pharmacological ingredients. Generally, these are based around compositions which contain bulking agents in the form of indigestible polysaccharides ("dietary fibre" and "resistant starch") and, as such, promote faecal bulk. It is known that increased intake of dietary fibre and, to a lesser extent, resistant starch increase stool mass and the frequency of defecation in humans. Many fibre-rich preparations relieve simple constipation promptly and effectively and they can be of value in the prevention and management of diverticular disease. In addition to increased faccal bulk, some of the bulking agents produce benefits through the production of short chain fatty acids (SCFA). These acids are produced by the fermentation of dietary carbohydrates by the colonic microflora. It appears that the provision of SCFA to the colonic mucosa is essential for the maintenance of its integrity and for the normal functioning of the organ. Also, SCFA appear to be essential for the prevention of non-infectious large bowel disease and their supply may be especially significant for the distal colon (where the incidence of these diseases is greater). Production of SCFA appears to predominate in the caecum and proximal colon where the supply of substrate is high. As digesta passes along the colon, SCFA availability falls through exhaustion of substrate coupled with absorption and utilisation of the acids.
Recent studies have shown that the major SCFA, specifically acetate, propionate and butyrate (but possibly others including succinate and formate) may have a number of effects. Collectively, they lower digesta pH-an effect which is thought to lower the risk of colonic cancer through limiting the absorption of toxic compounds such as amines. The three major SCFA stimulate fluid and electrolyte transport and so reduce the risk of diarrhoea. They promote the flow of blood to the colon so that tissue perfusion may be improved and also they stimulate muscular contraction. The major SCFA seem to stimulate colonocyte proliferation and butyrate is thought to promote a normal cell phenotype. The latter acid is a major metabolic fuel for colonocytes and a number of in vitro studies have suggested that it lowers the risk of colonic cancer by promoting DNA repair and inhibiting the growth of transformed cells. Thus butyrate would appear to be the most beneficial of the SCFA. Other studies have demonstrated a direct therapeutic benefit in that instillation into the colon of humans with ulcerative colitis leads to a prompt remission of the condition.
In addition to lowering the risk of non-infectious disease, dietary fibre and resistant starch may be of benefit to colonic health by stimulating the growth of non-pathogenic bacteria. Thus the conditions favouring SCFA production and in particular butyrate will have an additional advantage in lowering disease risk.
Whilst some of the bulking agents increase SCFA, this does not necessarily occur with all such agents. For example, one of the commonly used bulking agents, namely psyllium, can act to reduce the level of SCFA in the colon. However, because psyllium also has the effect of promoting a reduction of cholesterol in the blood stream, it is a preferred bulking agent in situations where blood cholesterol reduction is of importance.
Other bulking agents which may have a cholesterol lowering effect include guar gum, pectin, konjac, modified carbohydrates including hydroxy propyl cellulose, hydroxy propyl methyl cellulose, carboxy methyl cellulose and methyl cellulose. Again, whilst they may have a blood cholesterol lowering effect they generally do not act to significantly improve SCFA levels in the colon and in particular may suppress levels of butyrate the most important of the SCFA.
Ideally, the most desirable laxative compositions should be able to reduce blood cholesterol and promote presence of SCFA including butyrate in the colon whilst providing the bulk necessary for healthy laxation.